When planning a dose distribution to be applied to a patient during radiotherapy or radiosurgery, it is generally desirable to avoid associating high doses with tissue outside the target region. A known approach disclosed in EP 2 038 010 B1 includes penalizing irradiation of normal tissue based on the distance of corresponding voxels to the target region in a medical image of the anatomical setting. This, however, does not allow for selective optimization of tissue indices which are frequently used to describe the quality of a planned dose distribution.
An object of the invention therefore is to provide a method of determining a dose distribution which results in an optimized quality measure for the dose distribution regarding irradiation of tissue lying outside the target region.
Aspects of the present invention, examples and exemplary steps and their embodiments are disclosed in the following. Different advantageous features can be combined in accordance with the invention wherever technically expedient and feasible.
Exemplary Short Description of the Present Invention
In the following, a short description of the specific features of the present invention is given which shall not be understood to limit the invention only to the features or a combination of the features described in this section.
The disclosed method encompasses analysing medical image data and an associated predetermined irradiation dose distribution for the contribution of voxels representing normal tissue and their associated dose defined by the irradiation dose distribution to a tissue index describing the quality of the irradiation dose distribution with regard to the dose applied to normal tissue. On the basis of that contribution, the voxels representing normal tissue are each assigned a priority with which the voxel will be included in an optimization procedure for optimizing the irradiation dose distribution to minimise irradiation of normal tissue as far as possible.
General Description of the Present Invention
In this section, a description of the general features of the present invention is given for example by referring to possible embodiments of the invention.
In general, the invention reaches the aforementioned object by providing, in a first aspect, a computer-implemented method for determining a dose distribution for use in a medical procedure (for example radiotherapy or radiosurgery) involving irradiation of an anatomical structure of a patient's body with ionising radiation. The method comprises executing, on at least one processor of at least one computer, the following exemplary steps which are executed by the at least one processor.
In a (for example first) exemplary step, medical image data is acquired which describes (for example, represents or defines) a medical image of the anatomical structure, wherein the anatomical structure comprises both a target region (comprising for example tumour tissue) which defines a target of the irradiation and non-target tissue, wherein irradiation of the non-target tissue shall be avoided. The anatomical structure can be located in any body part and can comprise at least one of soft tissue (such as skin, brain matter or an internal organ) and hard tissue (such as bone tissue or cartilage). The non-target tissue is for example defined to lie in a spherical shell defined in the medical image around the target region. The non-target comprises (specifically, consists of) normal tissue which is tissue within the patient that is not supposed to be irradiated but still receives a certain amount of dose nevertheless. This unwanted tissue irradiation exists because it is technically not possible to have dose only in the target region (the planning treatment volume PTV) and no dose anywhere else. By definition, normal tissue is used to refer to tissue outside the PTV. The non-target tissue is generally disjoint from the target region and may but need not necessarily comprise specific risk regions (organs-at-risk, i.e. organs which must not be irradiated during the medical procedure if a particular risk to the patient is to be avoided). The non-target tissue is for example normal tissue. The medical image data may be patient-specific and have been generated by applying a medical imaging modality (such as magnetic resonance tomography or computed x-ray tomography, conventional x-ray imaging or ultrasound imaging) to the specific patient's anatomical structure. Alternatively, the medical imaging data may not be patient-specific, for example if the medical image data comprises (specifically, consists of) atlas data describing (for example, representing or defining) an image-based model of the anatomical structure which has been generated for example from medical images generated for a population of patients. The medical image data in one example is three-dimensional image data but may alternatively be two-dimensional image data (for example, if it has been generated by conventional x-ray imaging).
In a further (for example second) exemplary step, dose distribution data is acquired which describes (for example, represents or defines) an irradiation dose distribution which is spatially defined in the reference system (coordinate system) of the medical image of the anatomical structure. The dose distribution data is for example predetermined (i.e. the irradiation dose distribution is for example predetermined) and has been generated before execution of the disclosed method starts. However, the irradiation dose distribution may also be predetermined in the sense that it is the output of an iteration of the disclosed method, i.e. that it is the result of an optimization step. “Predetermined” in this context therefore means that the irradiation dose distribution is not calculated in this step but read as an input data set, which may also be input from a stored result of previous iteration of the optimization algorithm described below.
In a further (for example third) exemplary step, prioritization data is determined which describes (for example, represents or defines), for each image unit of the medical image describing (for example representing or defining) non-target tissue, a priority of that image unit for consideration during an optimization of the irradiation dose distribution described by the dose distribution data. The prioritization data is determined based on the medical image data and the dose distribution data. The term of image unit denotes a pixel or voxel, respectively, depending on the dimensionality of the medical image data.
The priority defines an influence of the dose values associated with the image units having the respective priority on determining a desired changed irradiation dose distribution by applying an optimization algorithm. The priority associated with an image unit defines for example the influence of that image unit on a cost function to be optimized by the aforementioned optimization. The influence (and therefore also the priority) is represented by for example a numeric value which correlates with the contribution of the image unit to the optimization result, wherein for example irradiation dose values assigned to image units representing non-target tissue and associated with an irradiation dose of more than a prescribed dose are considered to have a higher influence on determining the desired changed irradiation dose distribution. The prescribed dose defines a dose which shall be administered to the target region and generally is predetermined and used as an input parameter to the disclosed method.
In a first embodiment of this step, tissue index contribution data is acquired based on the medical image data such as by image analysis of the medical image data. The tissue index contribution data describes (for example, represents or defines), for each image unit of the medical image of the anatomical structure describing non-target tissue, a contribution of the irradiation dose assigned to that image unit to at least one tissue index describing the quality of the irradiation dose distribution. The contribution is defined for example based on prior knowledge such as expert knowledge applied by a physician or a medical physicist. The prioritization data is in this first embodiment determined further based on the tissue index contribution data.
The aforementioned tissue index may be at least one of the conformity index and the gradient index which are associated with the dose distribution data and the medical image data. The conformity index CI is defined for example as CI=(volume of the target region*volume of the target region or the non-target tissue having an assigned irradiation dose larger than 100% of the prescribed dose)/(volume of the target region having an assigned irradiation dose larger than 100% of the prescribed dose)2. The gradient index GI is defined for example as GI=(volume of the target region or the non-target tissue having an assigned irradiation dose larger than 50% of the prescribed dose)/(volume of the target region or the non-target tissue having an assigned irradiation dose larger than 100% of the prescribed dose). In the expressions for CI and GI, * is the arithmetic operator of multiplication and/is the arithmetic operator of division.
For example, an image unit associated with a dose representing a predetermined percentage of a prescribed dose is determined to have an effect on either the CI or the GI or neither the CI nor the GI. Depending on the determined effect, the image unit is assigned a numeric value representing the priority for that image unit.
In a second embodiment of this step, the prioritization data is determined based on determining (for example, by determining) the result of |Didesired−Diactual(ω)| or (Didesired−Diactual(ω))2. Didesired is a desired irradiation dose to be applied to the non-target tissue represented by the i-th image unit, and Diactual (ω) is the irradiation dose to be applied to the non-target tissue represented by the i-th image unit and described by the dose distribution data and is dependent on the machine configuration ω. Didesired is set such that image units associated with a higher priority are associated with a higher value of |Didesired−Diactual(ω)| or (Didesired−Diactual(ω))2, respectively (i.e. depending on whether the result of |Didesired−Diactual(ω)| or (Didesired−Diactual(ω))2 is used as a basis for determining the prioritization data). For example for a high-priority image unit Didesired is set to Didesired=ai Diactual (ω) and for a lower-priority image unit Didesired is set to Didesired=bi Diactual (ω), and ai, bi are numeric values with 0<ai<bi<1. Therefore, Didesired is set for the i-th image unit such that the priority is reflected in the cost function.
In a further (for example fourth) exemplary step, changed dose distribution data is determined which describes (for example, represents or defines) a changed irradiation dose distribution which is spatially defined in the reference system of the medical image of the anatomical structure. The changed dose distribution data is determined based on the dose distribution data and the prioritization data. The changed irradiation dose distribution is for example an optimized irradiation dose distribution (such as an optimization of the irradiation dose distribution described by the dose distribution data). Furthermore, the changed irradiation dose distribution is comprised in the output of at least a step (i.e. at least one incremental step or in the final result) of an optimization algorithm having the dose distribution data and the prioritization data as an input. For example, the tissue index is the conformity index and the gradient index and determining the changed dose distribution data involves minimising the conformity index and the gradient index in order to optimize the predetermined irradiation dose distribution.
For example, the changed dose distribution data is determined based on (specifically, by) minimising a cost function ƒ(ω)=ƒ0(ω)+ƒ1(ω). ƒ1(ω) describes (for example, represents or defines) the part of the cost function for the non-target tissue and is defined as ƒ1(ω)=Σi pi[Didesired−Diactual(ω)]2. ƒ0(ω) describes (for example, represents or defines) the part of the cost function for parts of the anatomical structure other than the non-target tissue. Furthermore, ƒ0(ω) may for example describe (for example, represent or define) irradiation parameters like the number of monitor units or the total movement of all collimator leaves of a collimator of an irradiation device usable for irradiating the anatomical structure with ionising treatment radiation. ω is a parameter defining the machine configuration of an irradiation apparatus to be used for irradiating the anatomical structure, pi is a numeric value defining the priority of the i-th image unit describing non-target tissue and for example assigned to the i-th image unit based on prior knowledge, Didesired is a desired irradiation dose to be applied to the non-target tissue represented by the i-th image unit, and Diactual(ω) is the irradiation dose to be applied to the non-target tissue represented by the i-th image unit and described by the dose distribution data and is dependent on the machine configuration ω, where the machine configuration is defined by for example at least one of: the table angles of a table for placement of the patient when conducting the medical procedure, the vertical angle of a gantry of an irradiation device for irradiating the anatomical structure with the ionising radiation, the collimator angle of a collimator (for example, a multi-leaf collimator) for collimating the beam of ionising radiation to be emitted by the irradiation device, a jaw configuration of the irradiation device (the jaw is a for example rectangular additional collimator which is coarser than the multi-leaf collimator), and the monitor units per control point. A monitor unit is the smallest unit of photon fluence that a radiation therapy device can produce. It is linked to a certain dose at a certain depth for a certain aperture size of the radiation therapy device in a certain quality assurance device. Monitor units are not directly linked to the dose in patients because the aperture can be highly modulated. Large monitor units are usually associated with small apertures and therefore more leakage and stray irradiation of the patient.
On the basis of the changed dose distribution data, treatment plan data describing (for example, representing or defining) a treatment plan for conducting the medical procedure is determined in one example of the disclosed method. The treatment plan defines further details of the medical procedure to be carried out on the patient such as number of irradiation fractions and time intervals between fractions.
In a second aspect, the invention is directed to a computer program which, when running on at least one processor (for example, a processor) of at least one computer (for example, a computer) or when loaded into at least one memory (for example, a memory) of at least one computer (for example, a computer), causes the at least one computer to perform the above-described method according to the first aspect.
In a third aspect, the invention is directed to a non-transitory computer-readable program storage medium on which the program according to the second aspect is stored.
In a fourth aspect, the invention is directed to at least one computer (for example, a computer), comprising at least one processor (for example, a processor) and at least one memory (for example, a memory), wherein the program according to the second aspect is running on the processor or is loaded into the memory, or wherein the at least one computer comprises the program storage medium according to the third aspect.
In a fifth aspect, the invention is directed to a system for controlling an irradiation therapy device for use in a medical procedure involving irradiation of an anatomical structure with ionising radiation. The system comprises:    a) the at least one computer according to the fourth aspect;    b) at least one electronic data storage device storing at least one database comprising the medical image data and the dose distribution data,            wherein the at least one computer is operably coupled to the at least one database for acquiring, from the at least one database, the medical image data and the dose distribution data; and            c) the irradiation therapy device which is configured to emit a beam of ionising treatment radiation,            wherein the computer is operatively coupled to the irradiation therapy device so as to effect emission of the treatment radiation by the irradiation therapy device based on the changed dose distribution data.        
It is within the scope of the present invention to combine one or more features of one or more embodiments or aspects of the invention in order to form a new embodiment wherever this is technically expedient and/or feasible. Specifically, a feature of one embodiment which has the same or a similar function to another feature of another embodiment can be exchanged with said other feature, and a feature of one embodiment which adds an additional function to another embodiment can for example be added to said other embodiment.